Three phase surge wave instabilities have been observed occurring spontaneously in flowlines at otherwise stable conditions (e.g. no changes in flowrate, pressure, composition or temperature) in a number of gas condensate aqueous three phase flowlines (examples are the Huldra-Heimdal, Snøhvit and Mikkel-Midgard fields on the Norwegian Continental shelf).
Field data of three phase surge wave instabilities in a gas condensate flowline have been presented at the Multiphase Production Technology Conference, 12-14 Jun. 2013, Cannes, France by Pettersen et al in the paper: Liquid Inventory and Three Phase Surge Wave Data from the Midgard Gas Condensate Fields in the North Sea. The liquid surges are observed topside at the Åsgard B floater where they arrive with variation in surge volumes and frequency (see FIG. 1). The analysis shows that the onset of these surge waves occurs well into the friction dominated flow regime (see FIG. 2). The start of accumulation of water coincides with the first occurrence of surge flow (see FIG. 3). In predictions it is seen that this corresponds to liquid accumulation in the highest pipe inclinations.
The most important effect of the surge instabilities at Åsgard B is the problems with the liquid handling due to the large amount of liquid arriving at the receiving facility during the surging. The hydrate inhibitor surge drum runs full (surge capacity ˜55 m3) for low rates. The surge volumes increase when reducing the production rate and a minimum flow rate for the flowline is determined by the liquid handling capacity. It is not possible to produce through the flowlines at lower rates than the minimum flow rate. When the pressure in the reservoir is depleted the flow rates in the lines are reduced. When the rate reaches the minimum flow rate the production has to be stopped. By reducing the minimum flow rate it is possible to produce more from the field. The minimum flow rate is experienced at relatively high production rate for the Midgard field. Without any counter actions a considerable amount of gas will be left in the reservoir.
One effective measure to prolong the production is to reduce the pressure in the flowline. With reduced pressure the gas expands and drags along the liquid and effectively reduces the start of liquid accumulation and the minimum flow rate. However, reduction of the receiving pressure at Åsgard B comes with a cost and there is a limit to how much one can reduce the topside pressure. For the Mikkel-Midgard fields it has been decided to install a subsea compression station to boost and prolong the production for many years by reducing the well head pressures. Still the pressure between the compression station and Åsgard B will be relatively high. It is expected that the total production will still be limited by the surge wave instabilities.
Removing the instabilities will also make it easier to operate the production both at Åsgard B and at the subsea compression station. For instance at Åsgard B, hydrate formation has been experienced during unstable surge wave flow. This is believed to be caused by the irregular arrival of hydrate inhibitor/water at Åsgard B. This is currently remedied at Åsgard B by the additional injection of hydrate inhibitor topside when surge instabilities are expected.
Three phase surge wave instabilities are also seen at the Huldra-Heimdal field, where they have caused liquid handling problems. They are also experienced at the Snøhvit field. But at Snøhvit the production is tied back into a large slug catcher onshore and presently problems are not experienced there. However, if in the future there is a need to install a subsea compression station at Snøhvit there will potentially be a need to handle these surges.
Other publications that discuss three phase surge waves in multiphase systems include: Hagesæther, L. et al., Flow-Assurance Modeling: Reality Check and Aspects of Transient Operations of Gas/Condensate Pipelines, SPE Projects, Facilities & Construction, Volume 2, Number 1, March 2007, pp. 1-17; Torpe, H. et al., Liquid Surge Handling at Åsgard by Model Predictive Control, 14th International Conference on Multiphase Production Technology, Cannes, 17-19 Jun. 2009; and Landsverk, G. S. et al., Multiphase Flow Behaviour at Snøhvit, 14th International Conference on Multiphase Production Technology, Cannes, 17-19 Jun. 2009
It is the aim of the present invention to address the problems of reduced gas production caused by surge wave instabilities in three phase gas condensate flowlines.